The invention relates to a process for concentrating solutions containing 1 to 10% by weight of salts and resins.
In the preparation of organic products, residual solutions which contain 1 to 10% by weight of salts and resins frequently arise in the reaction chamber or in the subsequent purification processes. These residues are reprocessed, on the one hand, in order to recover the product or the solvent and, on the other hand, to minimize the waste which must be incinerated.
Examples of products which frequently have to be subjected to such concentrating are chlorobenzene, tolamine, metamine, orthamine, alphamine, cresol, sodium benzothiazole, ciprofloxacin, polyphenylene sulphide, sodium methylencaprolactam, diisocyanates, benzothiazyl-2-sulphen-morpholide and benzothiazyl-2-dicyclohexyl-sulphenamide.
A feasible process principle here is the evaporation process, since the re-usable products in general have a substantially higher vapor pressure than the wastes. However, this raises considerable problems in the process sequence, since 90 to 99% by weight of the residual solution must be evaporated, the liquid phase becomes viscous/pasty towards the end of the process, the salt precipitates and leads to hard crusts on the heated walls and, with increasing concentration, the temperature rises, the heat transfer deteriorates and further polymerization can occur, which causes a further increase in viscosity, means a product loss and additionally intensifies the tendency of the solution to form crusts or thick pastes which will not flow easily.
Discontinuous processes in a stirred kettle and in the still (VDI-Nachrichten 1983, page 10) or ladle are general state of the art. In some cases, circulation evaporators are also provided upstream of the vessels, but the degree of concentrating in these is only low because of the large circulation. These discontinuous processes have quite a number of disadvantages, since a discontinuous procedure always involves high operating costs and pollutes the environment. The entire charge reaches the highly viscous phase at high temperatures at the same point in time. Large forces must be applied for thoroughly mixing the system. In spite of the thorough mixing, the heat transfer is very poor, which means a long residence time of the product in this state. Therefore, further polymerizations frequently occur in these types of apparatus and, together with the precipitated salt, they lead to intensive formation of crusts and frequently require removal of the crusts by mining techniques.
Finally, these are open distillation processes, that is to say the temperatures in the end phase are substantially higher than in a closed process.
Because of the many disadvantages in discontinuous operation, continuous processes are desired. In addition to the cost advantage, continuous processes also have the advantage that always only a small part of the product is in the critical state of the highly viscous phase at the time. Moreover, continuously operated types of apparatus are closed systems, so that the product temperatures are always lower than in discontinuously operated types of apparatus.
A continuously operating apparatus which is widely used for such purposes is the thin-layer evaporator (Chemische Rundschau 1982, page 19). In this case, the solution is charged at the top to the inside of a wide tube and centrally arranged wipers ensure that a film is formed on the wall. The solution evaporates as it runs downwards through the apparatus. The vapor is taken off at the upper end and the liquid product at the lower end of the tube. The wipers also ensure that no thick crusts can form on the heated tube walls. However, the thin-film evaporator also has some disadvantages, since the investment costs are high, it is susceptible to faults because of the rotating parts and product splashes deposit on the wiper blades and cake thereon, so that the apparatus must be cleaned at certain intervals.